Impact of elevated CO2 and O3 on insect-mediated ecosystem processes in a northern deciduous forest

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Rising concentrations of atmospheric CO{sub 2} and O{sub 3} are altering the structure and function of forest ecosystems. Herbivorous insects are the major consumers in temperate deciduous forests, with the capacity to dramatically alter tree growth (via outbreaks), forest community composition and ecosystem dynamics (e.g., nutrient cycling). Until recently, however, experimental quantification of the impacts of CO{sub 2} and O{sub 3} on canopy herbivore communities and rates of defoliation and nutrient flux has not been addressed. This research, conducted at the Aspen FACE (Free Air CO{sub 2} Enrichment) facility in northern Wisconsin, U.S.A., evaluated the independent and interactive effects of ...
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Description

Rising concentrations of atmospheric CO{sub 2} and O{sub 3} are altering the structure and function of forest ecosystems. Herbivorous insects are the major consumers in temperate deciduous forests, with the capacity to dramatically alter tree growth (via outbreaks), forest community composition and ecosystem dynamics (e.g., nutrient cycling). Until recently, however, experimental quantification of the impacts of CO{sub 2} and O{sub 3} on canopy herbivore communities and rates of defoliation and nutrient flux has not been addressed. This research, conducted at the Aspen FACE (Free Air CO{sub 2} Enrichment) facility in northern Wisconsin, U.S.A., evaluated the independent and interactive effects of CO{sub 2} and O{sub 3} on (1) the abundance and diversity of forest canopy insect communities, and (2) rates of insect herbivory and transfer of material (leaf greenfall and insect frass) from the canopy to the forest floor. Results of studies of individual insects revealed that elevated CO{sub 2} and O{sub 3} influence the performance of individual species of damaging insect pests, but the magnitude of impact is influenced by both insect species and their host tree species. Censuses of canopy insects showed that some species were positively affected, some negatively affected, and some not affected by elevated CO{sub 2} and O{sub 3}. Moreover, overall species diversity was generally not strongly affected by CO{sub 2} and O{sub 3}. In summary, the effects of CO{sub 2} and O{sub 3} on forest insects is highly variable among species and over time, and thus difficult to generalize across broad taxonomic groups. Estimates of foliar damage revealed that CO{sub 2} and O{sub 3} have pronounced effects on canopy damage by insect herbivores. Averaged over three years, foliar biomass lost to insect feeding increased 86% in high CO{sub 2} environments and decreased 12% in high O{sub 3} environments. The increases/decreases were greater for aspen than for birch, indicating that the selective pressure of insects will shift across tree species in forests of the future. Herbivore-mediated material (green leaf tissue, insect frass) transfer from the canopy to the forest floor increased 37% in elevated CO{sub 2} and decreased 21% in elevated O{sub 3}. Nitrogen transfers paralleled those results: 39% increase in elevated CO{sub 2} and 19% decrease in elevated O{sub 3}.

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